Interspecific hybrids have a spectrum of abnormal phenotypes roughly correlating with evolutionary separation which range from failure of embryonic genome activation /early preimplantation lethality to viability with male sterility. Recently the importance's of small silencing RNA's have been found to play a critical role in the oocyte and male germline for embryo development and gametogenesis. One of the roles these RNA's are thought to provide is the control of transposable elements (TE) which represent a significant percentage of the human and mouse genomes. However evidence exists that these regulatory RNA's are also important in regulating host genes and epigenetic modifications of the genome. One consequence of this understanding is to view interspecies hybrids as lacking in sufficient homology between the regulatory RNA's and paternal genome which can then lead to the spectrum of abnormal hybrid dysgenesis phenotypes. In this proposal we hope to test this possibility by restoring a greater degree of homology between the paternal nuclear compartment and the oocyte cytoplasmic compartment, including stored regulatory small molecules by creating the first true interspecies hybrids. True hybrids will possess equal, or nearly so, contributions of both nucleus and cytoplasm from both parental species. We have chosen to use Mus musculus: caroli or spretus hybrids and assess them for viability, male infertility and somatic growth/obesity. If successful this approach can be used to characterize other interspecies hybrids. These experiments lay the groundwork for an increased understanding of how early events after fertilization program the zygote genome and as such will inform epigenetic gene regulation, transposon regulation, the reprogramming of a somatic cell nucleus in animal cloning, and evolution/speciation.